Solid phase microextraction (SPME), first introduced by Arthur and Pawliszyn in 1990, is a solvent-free, cheap, fast and easy technique that integrates sampling, isolation and concentration of analytes in one single step. It is widely used to extract an array of volatile and semi-volatile organic compounds.
SPME is a sample preparation technique that extracts different kinds of analytes (including both volatile and non-volatile) from different kinds of liquid or gaseous media. In general, the quantity of analyte extracted by the coating is proportional to its concentration in the sample matrix.
SPME is used widely to detect components in a sample in very small and trace amounts. These include, for example, in food analysis, such as for residual contaminants (pesticides/herbicides), quality control, characterization and forensic analysis, in in-vivo analysis, such as diagnostic tests, pharmacokinetic studies, and drug bioavailability in environment samples, such as water or air for contaminants and pollutants, and in biologicals, such as body fluid analysis, and breath analysis.
In an SPME assembly, the coated fiber is inside a septum piercing needle. The septum of a vessel containing the sample is pieced and the coated fiber is extended to expose same to a liquid sample or to air above the sample (headspace). The coating is designed to retain target compounds.
The fiber is then retracted into the microtube. The septum associated with the injection port of a separation/analysis instrument, such as a gas-chromatography system (SPME-GC), is then pierced, the fiber reexposed, and the target compounds are desorbed into a gas stream to form a tight sample plug at the entrance of a relatively cold column. The fiber is then retracted and removed. Using temperature programming, the column temperature is ramped up, hence separating the analytes.
SPME can also be hyphenated to HPLC (High-performance liquid chromatography), CE (Capillary electrophoresis), and other analytical techniques. In case of SPME-LC, for example, the desorption of analytes is achieved via changing the polarity of the mobile phase.
A key component of the fiber, which is often made of silica or metal, is the coating. The coating in current commercial systems is often an absorbent. With absorbent coatings analyte molecules are taken up by the volume and become incorporated into a bulk phase. The coating may be a liquid phase with the absorbed analyte molecules in solution. Commonly used coatings include polydimethylsiloxane (PDMS), polyethylene glycol (PEG, carbowax) and polyacrylate.
Major problems with these coatings involve solvent incompatibilities that may lead to swelling in organic phases. The life of the fiber is often short, due to the fragile nature of the fiber substrate, especially if it is silica. Other problems include high cost, a limited number of compounds that can be extracted into the coating, and the relatively low thermal, and mechanical stability of the coatings. In addition, the coatings may not have strong adhesion to the substrate. This problem has been somewhat mitigated by applying adhesion or bonding layers between the substrate and extraction phase of the coating. However, these problems persist.
Another problem with absorptive coatings is carryover, where analyte is retained from previous uses of the fiber that may show up in subsequent analyses, and, therefore, compromise the results of the subsequent analyses. The desorb conditions are usually designed to desorb analyte as close to 100% as practically possible, but with some coatings significant analyte amounts are retained and carried over into the next sampling.
Adsorbent coatings, in contrast to absorbent coatings, involve retention of analyte molecules to a solid surface (not to the volume) of a solid. Effective adsorbents have a high surface area, which can be provided by solids with high porosity with small pores. Adsorbent coatings have been used in several applications. Solid adsorbent coatings include carbowax-divinylbenzene, PDMS-divinylbenzene, and carboxen-PDMS.
As solid coatings involve adsorption of analytes on a surface, the process is usually faster as compared to liquid coatings. On the flip-side, lower porosity and competitive adsorption may limit the extraction efficiencies of solid phases. Various techniques are now being used to make these solid phases, including sol-gel chemistry, electro-deposition, and attaching nanoparticles to the fiber using thermo-stable adhesives. All of these coatings suffer from various drawbacks, including very long preparation times, limited mechanical, thermal, and solvent stability, the ability to extract limited numbers of compounds, and short lifetimes of the fibers.
In commercial coatings such as CAR-PDMS, PDMS-DVB, particles with adsorptive surfaces have been embedded into absorptive coatings. While these may have modified sorptive properties, they suffer from similar problems as absorption coatings.
In general, there are various methods for applying coatings upon substrates, each with varying properties. One class of coating systems involves physical vapor deposition. This process involves depositing atoms or molecules in the vapor phase on a substrate. Examples include sputter deposition, electron beam evaporation, thermal evaporation, and pulsed laser deposition.
All physical vapor deposition systems require production of a precursor vapor material that then condenses upon and is retained on a substrate to form a coating. The vapor can be created by, for example, thermal evaporation, electron-beam, evaporation, sputtering, including DC sputtering and, RF sputtering, cathodic arc vaporization, laser ablation, decomposition of a chemical vapor precursor. All of these are contemplated by the present method.
At a relatively higher gas pressure, the ejected atoms from the target can impact with other atoms or molecules on its path and travel to the substrate diffusely, impacting the substrate from random directions. It has already been shown that if atoms impact the substrate obliquely, due to, for example, substrate orientation to the target or to higher pressure, “defects” in the dense coating can occur. At these conditions it has been observed that films with columnar grains of different densities will form.
A method known as glancing angle deposition (GLAD), involves exploitation of the shadowing effect to create deposited films with various properties. The flux of vapor relative to the substrate is oblique, which results in the growth of slanted columnar microstructures. By manipulating the direction of flux during deposition, various columnar shapes have been obtained, with a wide range of porosity.